1. Field of the Invention
This invention relates generally to a system and method for treating upper airway obstruction, sleep disordered breathing, upper airway resistance syndrome and snoring by manipulating the structures of the oropharynx, including the tongue and the palate.
2. Description of the Related Art
Respiratory disorders during sleep are recognized as a common disorder with significant clinical consequences. During the various stages of sleep, the human body exhibits different patterns of brain and muscle activity. In particular, the REM sleep stage is associated with reduced or irregular ventilatory responses to chemical and mechanical stimuli and a significant degree of muscle inhibition. This muscle inhibition may lead to relaxation of certain muscle groups, including but not limited to muscles that maintain the patency of the upper airways, and create a risk of airway obstruction during sleep. Because muscle relaxation narrows the lumen of the airway, greater inspiratory effort may be required to overcome airway resistance. This increased inspiratory effort paradoxically increases the degree of airway resistance and obstruction through a Bernoulli effect on the flaccid pharyngeal walls during REM sleep.
Obstructive Sleep Apnea (OSA) is a sleep disorder that affects up to 2 to 4% of the population in the United States. OSA is characterized by an intermittent cessation of airflow in the presence of continued inspiratory effort. When these obstructive episodes occur, an affected person will transiently arouse, regain muscle tone and reopen the airway. Because these arousal episodes typically occur 10 to 60 times per night, sleep fragmentation occurs which produces excessive daytime sleepiness. Some patients with OSA experience over 100 transient arousal episodes per hour.
In addition to sleep disruption, OSA may also lead to cardiovascular and pulmonary disease. Apnea episodes of 60 seconds or more have been shown to decrease the partial pressure of oxygen in the lung alveoli by as much as 35 to 50 mm Hg. Some studies suggest that increased catecholamine release in the body due to the low oxygen saturation causes increases in systemic arterial blood pressure, which in turn causes left ventricular hypertrophy and eventually left heart failure. OSA is also associated with pulmonary hypertension, which can result in right heart failure.
Radiographic studies have shown that the site of obstruction in OSA is isolated generally to the supralaryngeal airway, but the particular site of obstruction varies with each person and multiple sites may be involved. A small percentage of patients with OSA have obstructions in the nasopharynx caused by deviated septums or enlarged turbinates. These obstructions may be treated with septoplasty or turbinate reduction procedures, respectively. More commonly, the oropharynx and the hypopharynx are implicated as sites of obstruction in OSA. Some studies have reported that the occlusion begins with the tongue falling back in an anterior-posterior direction (A-P) to contact with the soft palate and posterior pharyngeal wall, followed by further occlusion of the lower pharyngeal airway in the hypopharynx. This etiology is consistent with the physical findings associated with OSA, including a large base of tongue, a large soft palate, shallow palatal arch and a narrow mandibular arch. Other studies, however, have suggested that increased compliance of the lateral walls of the pharynx contributes to airway collapse. In the hypopharynx, radiographic studies have reported that hypopharyngeal collapse is frequently caused by lateral narrowing of the pharyngeal airway, rather than narrowing in the A-P direction.
OSA is generally diagnosed by performing overnight polysomnography in a sleep laboratory. Polysomnography typically includes electroencephalography to measure the stages of sleep, an electro-oculogram to measure rapid eye movements, monitoring of respiratory effort through intercostal electromyography or piezoelectric belts, electrocardiograms to monitor for arrhythmias, measurement of nasal and/or oral airflow and pulse oximetry to measure oxygen saturation of the blood.
Following the diagnosis of OSA, some patients are prescribed weight loss programs as part of their treatment plan, because of the association between obesity and OSA. Weight loss may reduce the frequency of apnea in some patients, but weight loss and other behavioral changes are difficult to achieve and maintain. Therefore, other modalities have also been used in the treatment of OSA, including pharmaceuticals, non-invasive devices and surgery.
Among the pharmaceutical treatments, respiratory stimulants and drugs that reduce REM sleep have been tried in OSA. Progesterone, theophylline and acetozolamide have been used as respiratory stimulants, but each drug is associated with significant side effects and their efficacy in OSA is not well studied. Protriptyline, a tricyclic antidepressant that reduces the amount of REM sleep, has been shown to decrease the frequency of apnea episodes in severe OSA, but is associated with anti-cholinergic side effects such as impotence, dry mouth, urinary retention and constipation.
Notwithstanding the foregoing, there remains a need for improved methods and devices for treating obstructive sleep apnea.